Fix `assert_in_delta` test failure
Fix `assert_in_delta` test failure

`assert_in_delta` in `timestamp_test.rb` causes an intermittent test
failure. It looks like that caused by subseconds truncation in MySQL 5.5.

Example:

```
  1) Failure:
  TimestampTest#test_touching_many_attributes_updates_them [/home/travis/build/rails/rails/activerecord/test/cases/timestamp_test.rb:125]:
  Expected |2016-02-06 09:22:10 +0000 - 2016-02-06 09:22:09 UTC| (1.000837185) to be <= 1.
```

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